Largest living organism
Shark Bay Posidonia australis seagrass
20000 hectare(s)
Australia (Shark Bay)

The largest single living organism based on area is a specimen of Posidonia australis seagrass (aka Poseidon’s ribbon weed) located in Shark Bay off Western Australia, covering approximately 200 square kilometres (77 square miles) – equivalent to around 28,000 soccer fields or more than 450 times bigger than Vatican City, the world's smallest country. The superlative seagrass meadow was described in a paper published in Proceedings of the Royal Society B on 1 June 2022.

This claims the title of largest organism from a specimen of Armillaria ostoya honey mushroom – known colloquially as the "Humongous Fungus" – growing in the Malheur National Forest of Oregon, USA, which occupies 965 ha (2,385 acres). It retains its title as the world's largest fungus.

Also the largest plant by area, the seagrass meadow spans a distance of c. 180 km (112 mi) from White Island in Shark Bay's western gulf to the Faure Sill in the eastern gulf. Spawned from a single seed, based on this species' growth rate (15–35 cm/6 in–1 ft 1.8 in per year), it is estimated to be around 4,500 years old. It has spread over the millennia via underground clonal shoots known as rhizomes meaning that the entire plant is connected and shares the same DNA, though the authors of the study note that certain patches (ramets) in a clonal plant do become separated over time so gaps can emerge.

The meadow's extent is based on data preceding 2010–11 due to an unprecedented marine heatwave (MHW) that occurred in the austral summer of 2010–11 across the Western Australian coastline that led to a record seasonal reduction of 1,310 km2 (506 sq mi) of Shark Bay's seagrass. The MHW increased the temperature of the water by 2–5°C warmer than average.

Another record-breaking clonal plant is Pando, a network of quaking aspen (Populus tremuloides) growing in the Wasatch Mountains of Utah, USA, which is considered the world's most massive plant. The clonal forest, comprised of around 47,000 individual stems, was confirmed in December 1992 to be a single root system, covering 43 ha (106 acres) and weighing an estimated 6,000 tonnes (6,600 US tons). The clonal system is genetically uniform and acts as a single organism, with all the component trees (part of the willow family) changing colour or shedding leaves in unison.

The research was a collaborative study undertaken by the University of Western Australia and Flinders University (both Australia).